Application of Dry Fertilizers depends on % nutrient in fertilizer,
nutrient release rate or time period nutrients are released, PPM of nutrients
desired, the volume of the container. Most granular fertilizers have a 4 to 6 week longevity, Organics 1 to 4 months,
CRFs 4, 6, and 9 months are common. Nutrients concentrations are commonly based
to dry soil weight reference approx. 1.42 g/ml for loam. Some Potting soils have
fertilizer added. For Example MG Potting soil lists about 0.21% Nitrogen from 4
to 6 month CRF on the Bag. This is referenced to the dry weight of the mix
usually 0.24g/ml for most potting soils. So the MG contains per liter of mix
0.21/100*240g/Liter equals 0.5g N per liter. This is released in 4 to 6 months,
with a 5 month average 0.5g/5 = 0.1g Nitrogen per month (approx. 100ppm N , and
1.2gN per year). If you want to approximate
your liquid feed rate to dry fertilizers. Calculate the %N solution you use
times the water you apply monthly .
For example if you where applying 1/2tsp per gallon of 9%N weekly, and
assuming your mix has 50% WHC. You are applying 2 gallons of solution monthly per gallom of soil mix. This equals 1 tsp of 9%N per month( 1tsp 5g
* 9/100= 0.45gN) per each gallon of mix monthly.

PPM Based Dry Fertilizer Application based on Water density reference 1000g/liter

Amount of Fertilizer in Grams=(PPM desired/1E6)*( Volume of
Mix in Liters *1000)/(% Nutrient in fertilizer/100)

Example: if I want 100ppm N using a 13% N dry fertilizer, 4 to 6
week release rate for a #1 Nursery Container 3 Liter volume. I would use :

(100/1E6*3000g)/(13/100) = 2.3 g of fertilizer every 6 weeks approx. 1/2tsp.

Sprinkler Fertilizers of soil surface, the water in well.